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Role of the Malpighian tubules in acid-base regulation in the desert locust Schistocerca gregaria Stagg, Andrew Peter


Malpighian tubule fluid from Schistocerca gregaria adults, starved for one day, was collected by cannulation of the gut in situ, both before and after injecting 10 μmol of HCl or NaCl into the haemocoel. Haemolymph pH at the neck remained depressed by 0.3 units for at least 8 hours in HCl- compared with NaCl-injected locusts. Haemolymph pH remained lower compared with pre-injected haemolymph values for several hours. The pH of tubule fluid remained about 0.5 units more acid than haemolymph under all conditions. Thus, net tubular acid secretion was proportional to haemolymph acid-base status. The greater acidity of tubular fluid after acid-injection was associated with lower estimated bicarbonate concentrations and higher Pco₂, without any change in total CO₂ when compared with controls. The combined contribution of bicarbonate, phosphate, and urate to total buffer capacity of tubular fluid was estimated to be 75%, with bicarbonate responsible for 55% of the total. The maximum rate of acid removal by all Malpighian tubules of starved locusts, including H⁺ trapped in ammonium ions, was calculated to be very small in relation to the acid-load injected into the haemocoel. Ligation of the locust hindgut so as to prevent posterior Malpighian tubule fluid flow significantly lowered haemolymph pH as compared with either anterior or sham ligations after HCl injection. Injection of an artificial saline into the hindgut restored haemolymph pH after HCl injection. Thus the hindgut per se directly contributes to haemolymph pH recovery from acidosis. Increasing the temperature to 37° C from 21° C caused a 4-fold increase in tubular secretion rates. In addition, starved locusts maintained a temperature/haemolymph pH ratio that is consistent with the alphastat hypothesis under starved conditions, but temperature does not affect haemolymph pH under fed conditions. Feeding locusts at 21° C lowered haemolymph pH 0.28 units. At 37° C, feeding had no effect on haemolymph pH. Feeding did not initiate any changes in tubular fluid acid-base variables at either 21° C or 37° C. The buffering capacity of tubular fluid was not affected by temperature or feeding. Micropuncture of the gut indicated feeding initiated a movement of alkaline midgut contents posteriorly into the hindgut. The maximum rate of acid removal was calculated to double for starved locusts at 37° C as compared to 21° C, but feeding reduced net acid excretion to zero at both 21° C and 37° C.

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